Unit testing for def/ module.

Added unit tests to test code in def/ module.

vendor/golang.org/x/crypto/ssh/certs.go

@@ -222,6 +222,11 @@ type openSSHCertSigner struct {
 	signer Signer
 }
 
+type algorithmOpenSSHCertSigner struct {
+	*openSSHCertSigner
+	algorithmSigner AlgorithmSigner
+}
+
 // NewCertSigner returns a Signer that signs with the given Certificate, whose
 // private key is held by signer. It returns an error if the public key in cert
 // doesn't match the key used by signer.
@@ -230,7 +235,12 @@ func NewCertSigner(cert *Certificate, signer Signer) (Signer, error) {
 		return nil, errors.New("ssh: signer and cert have different public key")
 	}
 
-	return &openSSHCertSigner{cert, signer}, nil
+	if algorithmSigner, ok := signer.(AlgorithmSigner); ok {
+		return &algorithmOpenSSHCertSigner{
+			&openSSHCertSigner{cert, signer}, algorithmSigner}, nil
+	} else {
+		return &openSSHCertSigner{cert, signer}, nil
+	}
 }
 
 func (s *openSSHCertSigner) Sign(rand io.Reader, data []byte) (*Signature, error) {
@@ -241,6 +251,10 @@ func (s *openSSHCertSigner) PublicKey() PublicKey {
 	return s.pub
 }
 
+func (s *algorithmOpenSSHCertSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
+	return s.algorithmSigner.SignWithAlgorithm(rand, data, algorithm)
+}
+
 const sourceAddressCriticalOption = "source-address"
 
 // CertChecker does the work of verifying a certificate. Its methods